Suspension Geometry 101

phragle

Member
Feb 7, 2009
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Godfather to the blaster family
it's really not that difficult, and you can do most of it without math.

Say you want to create a long travel rear end. (desireable, since yamaha claims 7.1" of travel, take the spring off the shock and cycle the rear end thru the shock travel and your lucky if you see 5") to really accomplish this you need to move the swingarm shock mount foreward and down, but where to put it??

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1. take whatever rear shock you plan on using and measure from the middle of the top bolt hole (sticking in a bolt so it barely sticks out wil give you a better idea of the center of the hole, you need to be fairly precise, a 1/4" error will compound and really throw things off by the time your done) to the middle of the bottom hole and write this number down.

2. (this step is easier if the spring is off the shock) measure from the end of the shock body to the beginning of the bump stop, subtract 1/2~ 3/4" for bump stop compression (depending on the bumpstop)and write this number down.

3. subract measurement 2 from measurement 1 and write down. next put measurement 2 someplace far away so you dont screw something up.

4. you now have the extended and compressed length of your shock, take 2 pieces of say 1" or so square tubing and draw a line down the exact middle of one side on each one, then make 2 marks on the line, one for the top mount, one for the compressed length and on the other one a hole for the top mount and one for the extended length. Drill the top mount the exact size of the top mount. for the other two holes, find a pencil or sharpie etc that will fit sideways thru the swing arm. measure the diameter of the pencil and drill the other two holes that size. (you want a nice snug fit), cut off any excess tubing.

5. with the tires on and the front and rear shocks OFF the bike, set the bike down so the frame is sitting as low as it will go (depending on tire size it may hit the ground first) now block the frame about 3" off the ground (this will give you ground clearance when you bottom out) once you have your bottom out spot, take asharpie or a grease pencil and make a mark on the axle carrier and a mark on the frame straight up. measure and write the number down.

6. put the front shocks back on set the bike on its front wheels with a floor jack under the back of the frame and jack the back up untill its about even with the front or a little above (the chain should limit the downward travel of the swingarm at somepoint as it hits the chain roller...you can go farther, but you will have to relocatethe chain roller lower) you dont really want to exceed a 30 degree angle from horizontal or you will end up with chain torq issues (When you gas it, the back wheels try to go under the bike lifting the nack of the bike not the front). once you have the back jacked up where you want measure between the 2 marks (frame and carrier) that you made and write that down.

7. now comes the fun part, subtract the smaller number from the larger number. this will tell you how much travel is possible, next decide how much travel you want (probably 10~12" depending on what type of riding you are doing) and factor in variables, such as if you have 22" back tires and you think you want to put 20's or 18"s on it someday (the smaller the tire, the less travel you can have) and adjust your bottom out number accordingly.


8. put the bike on some kind of stand, remove the back fender if you havent allready so you have room to work and the back wheels. take a tie down between the grab bar and the carrier and lift the swingarm you have the swingarm at the measurement (between the marks you made on the frame and carrier) for your bottom out position. now take a piece of posterboard of light cardboard, cut it so it fits inside your swingarm lengthwise and so that it extends a couple inches under the swinger. now bolt the short piece of square tubing to the top shock mount using some washers as spacers so that you can swing it front and backwords, but it doesnt wobble side to side. place the pencil in the pencil hold and swing the bottom of tubing front to back using the pencil to draw an arc on the posterboard. leave the posterboard in place, but remove the tubing.

9. now using the tie down, lower the back of the swingarm however much travel you want. if you calculated out that 11" were posible and you want 11" lowerit 11". bolt the long piece of tubing to the top mount like you did before, swing it back and forth drawing another arc.

10. Where the 2 arcs intersect is where you need to locate the new shock mount, now "blueprint" the swingarm, plotting where you want that mark to be.

11. repeat the whole arc process a feww times and compare it with your "blueprint" to make sure you didn't screw something up. step back and think about if thereis enough clearance in this new location for the shock and spring, then the creative part of making the new mount is up to you. you can make a piece that bolts to the shock then to the swingarm, or make holes in the swingarm and weld in tubes etc. but the odds are the new mount will be somewhere between the rails of the swinger resulting in some kind of creative mounting.

it's VERY easy to DOUBLE the travel on the back of the blaster.
 
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i see why the 450s and other quads use that different bottom shock mount design, the arms instead of directly onto the swingarm.


good right up. i may have to do this.

thanks.
 
linkages may have gotten better in the last couple years..I dont know. the linkages were never worth a damn in the desert. the fancy no-link 250 swingarm Roll sells for the 250r evolved from a bolt on gizmo Fisk made. (out of santee ca, he was one of the first a-arm makers too) the gizmo was called a "link eliminator" It bolted to the linkage mount on the swingarm on top and to front skidplate mounts on the bottom. the shock mounted to it providing 12" of travel and eliminating the linkage. stock shock worked great, though you needed to swap out 1 shim in the rebound stack.

Im sure there were also several made at C&J in fallbrook.(san diego) this company made aftermarkt frames for triumphs etc. and was also the first company to make aftermarket frames for the 250r and banshee (the banshee frame used 250r front end geometry to eliminat the 'j' arm) and the rear used a no link swingarm with the shock offset to one side. there was a kid who worked there...started doing side projects and eventually started his own shop across the parking lot (I think you have heard of him, Doug Roll of Roll designs)
 
Awesome How-to. What shock do you suggest using for 12" of travel. I kinda don't think the stock shock can hack it.
 
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phragle, the suspension force is with you....please enlighten me on what the benefit of the 88 shaft/piston in the 86 body nets you
 
Has anybody ever done a photo diy on this? I think I understand most of it, but pictures would definitely be helpful.
 
Ok, im looking into this and here is the part i dont understand...

4. you now have the extended and compressed length of your shock, take 2 pieces of say 1" or so square tubing and draw a line down the exact middle of one side on each one, then make 2 marks on the line, one for the top mount, one for the compressed length and on the other one a hole for the top mount and one for the extended length. Drill the top mount the exact size of the top mount. for the other two holes, find a pencil or sharpie etc that will fit sideways thru the swing arm. measure the diameter of the pencil and drill the other two holes that size. (you want a nice snug fit), cut off any excess tubing.

I know Phragle has since moved on so maybe someone else can help me here...when it comes to marking the 1inch square tubing: tube one has a mark for the top mount and the compressed length and Tube Two has a mark for the top mount and the extended length...correct?

Now is there a specific distance between the top mount and compressed mark on tube one and another specific distance between top mount and extended mark?

if so, what would this be? and how does it work on a 1inch piece of tubing?

OR, are the holes just randomly spaced and the same for both tubes?
 
If I understood it correctly you're making a "compass" of sorts, compressed and extended radius. Only reason I can see for the tube is so it doesn't flop around as much? I was just going to use flat stock when I get to that point. YZ Otis has a post on how he calculated compressed length without removing spring.
 
If I understood it correctly you're making a "compass" of sorts, compressed and extended radius. Only reason I can see for the tube is so it doesn't flop around as much? I was just going to use flat stock when I get to that point. YZ Otis has a post on how he calculated compressed length without removing spring.

I just used the shock in place of the square tubes. Took the spring off the shock ( needed paint anyway) Tied it off in the compressed position with the bumper compressed about 20mm. Wrapped masking tape around a short pencil till it fit the bottom shock bolt hole and made my arc. This is with the arm in the position you want it at bottom of travel.
If you have the spring off the shock you can easily check your results after you tack up the arm.